The invention lies in the field of semiconductor memory fabrication and relates to a method for fabricating an insulation collar in a trench capacitor.
Volatile semiconductor memories (RAM) generally have a storage capacitor and a selection transistor in each individual memory cell. The storage capacitor can either be formed as a so-called stacked capacitor above the silicon substrate or be introduced in the form of a depression extending into the semiconductor substrate. This depression generally has an aspect ratio of greater than 20:1 and is usually referred to as a trench by experts.
A memory cell of that is described in U.S. Pat. No. 5,909,044, for example. In the case of that memory cell, the conductive filling of the trench, which forms the inner electrode of the capacitor, is insulated from the active regions of the assigned selection transistor by means of an insulation collar in the upper region of the trench. In accordance with U.S. Pat. No. 5,909,044 the collar is formed by thermal oxidation of the semiconductor substrate in the upper region of the trench after partial filling of the trench with polysilicon. Valuable semiconductor substrate area is consumed in the process. In order to minimize the area consumed, in the method mentioned it is necessary for the trenches to be formed correspondingly narrower. However, this increases the aspect ratio of the trenches, which are thus more difficult to etch. Furthermore, an undesirable so-called bird""s beak can form during the oxidation during the transition from the insulation collar to the adjoining dielectric.
The same problems also occur in the case of the method in accordance with U.S. Pat. No. 5,937,292, since the insulation collar is formed by thermal oxidation in that case, as well.
In accordance with a further method described in in U.S. Pat. Nos. 6,008,104 and 6,310,375 (European patent application EP 0 949 680 A2), the insulation collar is formed by conformal deposition of an insulating layer with subsequent anisotropic etching in the upper region of the collar. In this case, the insulating layer is deposited onto the trench, which is partly filled with a sacrificial filling material. Since the sacrificial filling material must be removed again from the trench after the deposition of the insulating layer, it is necessary to form an opening in the insulating layer toward the sacrificial filling material. This opening is formed by the aforementioned anisotropic etching. Since the insulating layer must have a certain thickness in order to be able to serve as an insulation layer, the cross section of the trench is greatly constricted by the insulating layer. Moreover, the insulating layer often has a larger thickness on the sacrificial filling material than on the sidewalls of the trench, so that the opening to be formed can only be formed with a major overetching of the insulating layer. In this case, it can happen, however, that the insulating layer is etched back from the upper edge of the semiconductor material by the anisotropic etching and, consequently, is no longer present there as insulation layer. Therefore, the method described therein is not suitable for fabricating trench capacitors with a small cross section. The same problems also occur in European application EP 0 949 684 A2 and U.S. Pat. No. 5,945,704.
It is accordingly an object of the invention to provide a method of fabricating an insulation coller in a trench capacitor, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which ensures the formation of a reliable insulation collar, even when the trench capacitor has a small cross section.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of fabricating an insulation collar in a trench capacitor, which comprises the steps of:
a) providing a semiconductor substrate;
b) forming a trench in the semiconductor substrate;
c) forming an insulating layer in the trench in order to form an insulation collar;
d) subsequently filling a lower region of the trench, the lower region extending up to a predetermined height, with a sacrificial filling material, so that an upper region of the trench, the upper region extending above the predetermined height, remains uncovered by the sacrificial filling material;
e) forming a patterning layer that is relatively thin in comparison with the insulating layer on the insulating layer and on the sacrificial filling material in the trench;
f) forming an opening in the patterning layer toward the sacrificial filling material, the patterning layer remaining on the insulating layer in the upper region of the trench;
g) subsequently removing the sacrificial filling material from the trench; and
h) removing the insulating layer from the lower region of the trench by etching the insulating layer selectively with respect to the patterning layer, so that the insulation collar is produced in the upper region of the trench.
It will be seen that the deposition of the insulating layer, from which the insulation collar is formed, before the trench is partly filled with a sacrificial filling material, lies at the heart of the invention. As a result, firstly an insulating layer having an approximately uniform thickness is formed on the sidewalls of the trench. This is followed by the partial filling of the trench with a sacrificial filling material up to a predetermined height. This height approximately defines the boundary region between an upper and a lower region of the trench. After filling with the sacrificial filling material, a usually thin patterning layer is deposited in the upper region of the trench and an opening toward the sacrificial filling material is formed in said layer. In this case, however, the S insulating layer continues to be covered by the patterning layer. The patterning layer is preferably formed such that it is significantly thinner than the insulating layer, so that its thickness on the sacrificial filling material is likewise significantly smaller. As a result, this patterning layer can also be removed relatively easily from the sacrificial filling material. Major overetching is not necessary here. Once the opening has been formed in the patterning layer, the sacrificial filling material is removed from the trench. This can be done for example by means of a wet-chemical etching selectively with respect to the patterning layer. During the removal of the sacrificial filling material, the relatively thick insulating layer also simultaneously serves as an etching stop layer and thus protects the semiconductor substrate from an attack by the etching chemicals. Isotropic dry-chemical removal of the sacrificial layer selectively with respect to the insulating layer and patterning layer is also possible (e.g. using SF6 in the case of amorphous silicon or polysilicon as sacrificial filling material). After the removal of the sacrificial filling material, the insulating layer is uncovered in the lower region of the trench and can be removed by etching selectively with respect to the patterning layer. In this case, the insulating layer remains only in the upper region of the trench and forms the insulation collar there.
With the method according to the invention, the problems which occur in U.S. Pat. Nos. 6,008,104 and 6,310,375 (EP 0 949 680 A2) are avoided, so that even trench capacitors having a significantly smaller cross section can be fabricated. Therefore, this method can also be employed with structure widths (cross section) of xe2x89xa6100 nm.
A further advantage is that the insulating layer is firstly applied completely and then removed by etching from the lower region of the trench. This produces very smooth transitions from the insulation collar to the lower region of the trench, which facilitate the filling of the entire trench with a conductive material.
The insulating layer and the sacrificial filling material should preferably be composed in each case of a material which can be etched selectively with respect to the material of the patterning layer. Furthermore, it is favorable if the sacrificial filling material can also be etched selectively with respect to the insulating layer. As a result, the insulating layer can act as an etching stop layer during the removal of the sacrificial filling material.
The fabrication of the trench capacitor is advantageously concluded, after the formation of the insulation collar, by the trench being lined with a dielectric and finally being filled with a conductive material in order to form a further electrode of the trench capacitor. The other electrode is formed by the semiconductor substrate in this case. Said substrate can be suitably doped before the lining of the trench with a dielectric.
In order to enlarge the capacitor area, the lower region of the trench is suitably enlarged after the formation of the insulation collar by an etching strip, so that the trench capacitor has a bottle shape in longitudinal section.
Furthermore, it is preferred that a thermal oxide is formed before the deposition of the insulating layer on the sidewalls of the trench. Said thermal oxide has only a relatively small thickness, so that hardly any semiconductor material is consumed.
It is also possible to form the thin thermal oxide after deposition of the insulating layer below the latter by means of oxidation through the insulating layer. Furthermore, a thermal annealing step is favorable, by means of which the insulating layer is densified and the interfaces to the thermal oxide and to the semiconductor substrate are sealed. This reduces leakage currents from the outer electrode of the trench capacitor to the selection transistor along the insulating layer.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating an insulation collar in a trench capacitor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.